Spectroscopy
Reading light to investigate composition, temperature and atmospheres.
Each species removes light at its own set of wavelengths. Toggle one on and dark gaps appear in the dispersed light — the pattern, not the colour, is what identifies what the light passed through.
When light is spread out by wavelength, it is not smooth. Specific wavelengths are missing or enhanced, forming a pattern of dark and bright lines.
- 01Each chemical species absorbs and emits at a characteristic set of wavelengths.
- 02The pattern present identifies which species the light passed through or came from.
- 03Line strengths and shapes carry information about temperature, pressure and motion.
- Observation
A spectrum with dark absorption features at particular wavelengths.
- Measurement
Wavelength positions, depths and widths of the features.
- Physical model
Atoms and molecules absorb photons only at energies matching their internal transitions.
- Inference
Which species are present, and under what broad conditions.
- +Presence of specific atoms and molecules
- +Broad temperature and pressure conditions
- +Line-of-sight motion, through Doppler shift
- +Relative abundances, within a modelled atmosphere
What a method cannot settle matters as much as what it can. Each note below is tagged by how firm the statement is.
- INFERENCE
A spectrum reports the layers the light actually passed through. Deeper or cloud-covered regions can be hidden entirely.
- HYPOTHESIS
Converting line depths into abundances requires an atmospheric model. Different reasonable models can fit the same spectrum with different compositions.
- OBSERVATION
For faint or distant targets, the signal is small compared with instrumental and stellar noise, so weak features remain contested.
Exoplanet atmospheres
During a transit, a thin sliver of starlight filters through the planet's upper atmosphere. Comparing in-transit and out-of-transit spectra isolates what that atmosphere absorbed.
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